With the advancements of modern power electronics, the switched reluctance machine with its simple rugged design allowing high speed operation in harsh environments is beginning to be used in an ever widening variety of aerospace applications. The typical switched reluctance machine 18, as shown in cross section in FIG. 1, is an electronically controlled stepping motor that has salient poles on both the stator 48 (51, 53, 54, 55, 56, and 57) and rotor 20 (22, 24, 26, and 28). The stator 48 is provided with concentrated excitation windings 43, 45, 47, 49, 50, and 52 where the diametrically opposite poles 43 & 47, 45 & 49, and 50 & 52 are connected in series or in parallel to form an individual phase. FIG. 2 illustrates a typical series connection with associated drive circuitry including transistors 37 and 39, and diodes 33 and 35, and FIG. 3 a typical parallel connection. Various control methods exist for the switched reluctance machine to allow operation as a starter-generator, a variable speed motor drive, and in an actuator drive system to name but a few common examples.
The switched reluctance machine operates by energizing a stator pole pair 50 & 52 to create equal magnetomotive forces prior to alignment with the rotor poles 24 and 28 (see FIG. 1). This magnetomotive force produces a symmetrical flux distribution 58, and thus symmetrical magnetic forces which pull the rotor poles 24 and 28 into alignment with the energized stator poles 54 and 56. As the stator and rotor poles come into alignment (28 with 54, and 24 with 56), the stator pole pair is de-energized and the next pole pair is energized to continue the rotor rotation. Under normal operation, the flux distribution 58 is symmetrical due to the balance of the ampere-turns of the phase coils 50 and 52 for each stator pole pair 54 & 56 as illustrated schematically in FIG. 1, and graphically in FIG. 4 which shows the variation of flux across the air gap 44, .phi..sub.44, and 46, .phi..sub.46, of FIG. 1 as the rotor poles 24 and 28 rotate into and out of alignment with the energized stator poles 54 and 56.
If, however, one of the phase coils 50 (see FIG. 5) of the stator pole pair 54 & 56 has shorted turns (for the series connection of FIG. 2 or the parallel connection of FIG. 3), or if it were to become open circuited (for the parallel connection of FIG. 3), the magnetomotive forces would no longer be balanced, and the flux distribution 58 would lose its symmetry across air gaps 44 and 46 as illustrated schematically in FIG. 5 and graphically in FIG. 6. Under such conditions the switched reluctance machine's rotor 20 is subject to asymmetrical forces due to the difference of magnetic pull between the opposite poles of the faulted phase. These resultant forces are very high and periodic, and can lead to a total machine failure, especially when the switched reluctance machine has inherent mechanical imbalance. This problem is recognized and discussed by T. J. E. Miller of the SPEED Laboratory, University of Glasgow, in his paper entitled "FAULTS AND UNBALANCE FORCES IN THE SWITCHED RELUCTANCE MACHINE" presented at the IEEE-IAS 281th Annual Meeting, in Toronto, on Oct. 3-8, 1993. In this paper the use of differential voltage sensing, with search coils 5 and 7 (see FIG. 7) or without search coils (see FIG. 8), is proposed to detect these faults by sensing a differential voltage Vxy (see FIG. 7) or Vuv (see FIG. 8), and isolate the controller from them to avoid machine failure. While this method may be sufficient to avoid machine failure by shutting the switch reluctance machine down, it may not be suitable where continued operation of the machine is required.
The instant invention is directed to overcoming the above problems associated with a faulted stator phase-coil while still allowing operation of the switched reluctance machine.